1. Field of the Invention
The present invention relates to a piezoelectric device, a process for producing the piezoelectric device, and a liquid discharge device using the piezoelectric device.
2. Description of the Related Art
Currently, piezoelectric devices constituted by a piezoelectric layer and upper and lower electrode layers are used, for example, as actuators installed in inkjet recording heads. In the piezoelectric devices, the piezoelectric layer expands and contracts in correspondence with increase and decrease in the strength of an electric field applied from the upper and lower electrode layers to the piezoelectric layer.
The inkjet recording heads have a basic structure in which a piezoelectric device as above and a diaphragm are attached to an ink-nozzle member. The ink-nozzle member has a plurality of ink chambers and a plurality of ink-discharge outlets. The ink chambers reserve ink, and the ink is externally discharged from the ink chambers through the ink-discharge outlets. Normally, the piezoelectric layer is patterned into separate pieces corresponding to the plurality of ink chambers.
The perovskite oxides such as PZT-based oxides are known as materials suitable for the piezoelectric layer, where PZT stands for lead titanate zirconate. The PZT-based piezoelectric layer can be formed, for example, by vapor phase epitaxy such as sputtering. Depending on the composition and the material of the dopant, some types of PZT-based piezoelectric layers formed by vapor phase epitaxy such as sputtering normally have spontaneous polarization oriented upward (i.e., the negatively-polarized side of the PZT-based piezoelectric layer is the lower side, and the positively-polarized side of the PZT-based piezoelectric layer is the upper side) unless special polarization processing is performed immediately after the formation of the PZT-based piezoelectric layers.
As disclosed in Japanese Unexamined Patent Publication No. 2005-209912, noble metals such as iridium (Ir) and platinum (Pt) are widely used in the lower electrode layers of the conventional piezoelectric devices. However, the noble metals such as Ir and platinum Pt are expensive. That is, use of the noble metals is not preferable from the viewpoint of the cost. In addition, the noble metals such as Ir are uneasy to etch. Specifically, the noble metals such as Ir cannot be etched by wet etching, and can be etched by only ion etching among various types of dry etching, so that patterning of the noble metals such as Ir is difficult.
Therefore, conventionally, the lower electrode layer is not patterned, and is formed to have a uniform film structure, and each of the PZT-based piezoelectric layer and the upper electrode layer is patterned into separate pieces corresponding to a plurality of ink chambers. In this case, it is necessary that the lower electrode layer be a grand electrode (to which a fixed voltage is applied), and the upper electrode layer realize address electrodes (to which variable voltages are applied). The drivers for driving the piezoelectric devices are usually positive-output drivers. Thus, conventionally, polarization-inversion processing is performed in order to make the spontaneous polarization oriented downward (i.e., so that the negatively-polarized side of the piezoelectric layer becomes the upper side, and the positively-polarized side of the piezoelectric layer becomes the lower side) before the upper electrode layer is driven by positive-output drivers.
However, in the case where a PZT-based piezoelectric layer has upward spontaneous polarization after formation of the PZT-based piezoelectric layer unless special polarization processing is performed, it is inefficient to perform polarization-inversion processing of the PZT-based piezoelectric layer, and the polarization-inversion processing may not be able to realize sufficient piezoelectric performance which the PZT-based piezoelectric layer can intrinsically exhibit. In view of the above circumstances, the present inventors consider that if the lower electrode layer can be patterned into address electrodes and the address electrodes can be driven by positive-output drivers, the polarization-inversion processing of the PZT-based piezoelectric layer can be dispensed with, and it is possible to achieve the sufficient piezoelectric performance which the PZT-based piezoelectric layer can intrinsically exhibit.